Measurements of the acoustic quality factor (reciprocal internal friction) of commercial aluminum alloys 2090, 5056, and 6061 in a torsional vibrational mode at a frequency of 1 kHz in the temperature range from 50 mK to 300 K are reported. The temperature dependence of the mode frequency, which is a measure of the shear modulus of the material, is also given. The quality factor (Q) of all alloys exhibited the highest values at the lowest measurement temperature. A heat‐treated sample of the 5056 alloy gave the highestQof 6.5×107at 53 mK. MaximumQvalues of 8.9×106in 2090 and 6.5×106in 6061 were observed. Internal friction mechanisms which may explain the measuredQvalues are discussed. The results provide support for the planned construction and operation of very sensitive 50‐mK aluminum‐alloy bar gravity‐wave detectors.